In an electron gun or an ion gun of a charged particle beam apparatus such as an electron microscope or a focused ion beam processing apparatus, particularly in an electron gun (or an ion gun) which generates a charged particle beam having high acceleration energy, an electron gun (or an ion gun) having the accelerating tube structure has been popularly used for applying a high voltage in a stable manner.
FIG. 2 is a schematic view of an electron gun having the accelerating tube structure. Assume an FE electron gun which uses an accelerating tube of six stages shown in the drawing. In the drawing, reference numeral 1 indicates an FE electron source whose distal end is sharpened to a radius of approximately several 100 nm, and reference numeral 2 indicates an extracting electrode. The accelerating tube 2 is formed by alternately stacking an insulating-material insulator 6 made of glass or ceramic and a conductor metal 9. An electrode is mounted on the respective conductive metals 9. The electrode arranged below the extracting electrode particularly constitutes a control electrode 5, and other electrodes constitute accelerating electrodes 10. A bleeder resistor 11 of several GΩ is mounted between the respective electrodes of the accelerating tube.
In operating the electron gun, a negative potential of several 100 kV is applied to the FE electron source 1. This voltage is particularly referred to as an accelerating voltage (V0). A positive extracting voltage (V1) of several kV is applied to the extracting electrode 3 relative to the FE electron source 1. An intense electric field is generated on the distal end of the FE electron source 1 by the extracting voltage (V1) so that an electron beam 4 is discharged from the distal end by a tunneling effect. A trajectory of the discharged electron beam 4 is controlled by the control electrode 5, and the discharged electron beam 4 is accelerated to energy of the accelerating voltage (V0) applied to the electron source while passing through the accelerating tube. Assuming the accelerating voltage as V0 and the control electrode as V2, a voltage between the control electrode and a ground voltage is expressed as V0-V2. When the bleeder resistors 11 between the respective stages have the same resistance, each inter-stage voltage becomes equal to (V0-V2)/5. Accordingly, a change in potential gradient in the accelerating tube becomes small. As a result, with the provision of the accelerating tube structure, an electron beam can be accelerated while suppressing the influence of an electrostatic lens effect at a low level.
As described above, the conventional accelerating tube uses the insulating-material insulator 6 made of glass or ceramic. When reflected electrons, scattered electrons or the like impinge on the insulating material, there exists a possibility that a charge is accumulated (a charge is increased) on a surface of the insulating material. When the charge on the surface of the insulating-material insulator 6 is increased, the potential distribution in the accelerating tube is changed so that a trajectory of the electron beam 4 is changed thus giving rise to a phenomenon that the electron beam flickers when the electron beam is observed. To prevent such a charge-up phenomenon, the conventional accelerating tube 2 has the structure where the accelerating electrode 10 has a complicated shape for avoiding the direct contact of reflected electrons and scattered electrons of the electron beam 4 with the insulating-material insulator 6. Such an accelerating electrode 10 makes the manufacture thereof complicated, and when the accelerating electrode 10 is assembled into the accelerating tube 2, a potential gradient on a center axis becomes unfixed strictly speaking so that the aberration of the electron beam 4 is also increased.
Recently, there has been reported a technique on a conductive insulator where the resistance of a surface of insulating ceramic is slightly lowered by injecting (doping) a foreign material (dopant) such as titanium carbide into the surface of the insulating ceramic. FIG. 3 shows one example of an electron gun where such a conductive insulator is used between a second stage and a sixth stage of the accelerating tube. A conductive insulator 7 used in the accelerating tube 2 is particularly characterized in that the resistance on an inner surface is made small. A specific resistance value falls within a range of several 100MΩ to several 10GΩ. With the use of the conductive insulator 7, a charge which is generated when the electron beam 4 impinges on the surface of the insulator flows as an electric current on the surface of the conductive insulator and hence, the charge is not increased. Accordingly, a shape of the accelerating electrode 10 on each stage can have the simple structure different from conventional electron guns. Further, by setting the resistance of the conductive insulator 7 used on each stage equal to a value of the conventional bleeder resistance, it is also possible to provide an electron gun where the bleeder resistance is eliminated.
In the accelerating tubes shown in FIG. 2 and FIG. 3, a voltage is divided corresponding to respective stages such that a potential gradient becomes as uniform as possible. However, with the use of the conductive insulator 7, even when the second stage and stages succeeding the second stage are collected into one stage and all accelerating electrodes 10 below the control electrode 5 are removed, a charge is not increased whereby the potential gradient at or below the control electrode 5 can be made uniform. FIG. 4 shows the schematic constitution of an accelerating tube having no electrodes at the position of the control electrode 5 or below. The accelerating tube of this electron gun is formed into an integral body and hence, high coaxiality can be maintained. Further, the accelerating tube can maintain a large diameter and hence, the gradient of the potential distribution in the vertical direction can be maintained uniformly. Accordingly, an electron beam 4 whose trajectory is controlled by the control electrode 5 straightly passes through the inside of the accelerating tube, and is discharged from the electron gun. As a result, the electron gun shown in FIG. 4 can simplify the internal structure thereof, and the aberration of the electron beam 4 can be made also small.
Further, FIG. 5 is an electron gun where a control electrode 5 is eliminated so that the whole accelerating tube 2 is formed of a conductive insulator 7. In this case, although a trajectory of an electron beam 4 extracted from an FE electron source 1 cannot be controlled, the electron beam 4 is accelerated straightly and is discharged from the electron gun and hence, the aberration is made smaller. Further, it is also possible to eliminate a control power source which supplies a control voltage (V2).